﻿// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.CodeDom;

namespace System.ComponentModel.Design.Serialization;

/// <summary>
///  An expression context is an object that is placed on the context stack and contains the most
///  relevant expression during serialization. For example, take the following statement:
///  button1.Text = "Hello";
///  During serialization several serializers will be responsible for creating this single statement.
///  One of those serializers will be responsible for writing "Hello".
///  There are times when that serializer may need to know the context in which it is creating its expression.
///  In the above example, this isn't needed, but take this slightly modified example:
///  button1.Text = rm.GetString("button1_Text");
///  Here, the serializer responsible for writing the resource expression needs to know the names of the target objects.
///  The ExpressionContext class can be used for this. As each serializer creates an expression and
///  invokes a serializer to handle a smaller part of the statement as a whole,
///  the serializer pushes an expression context on the context stack.
///  Each expression context has a parent property that locates the next expression context on the stack,
///  which provides a way for easy traversal.
/// </summary>
public sealed class ExpressionContext
{
    public ExpressionContext(CodeExpression expression, Type expressionType, object owner, object? presetValue)
    {
        Expression = expression.OrThrowIfNull();
        ExpressionType = expressionType.OrThrowIfNull();
        Owner = owner.OrThrowIfNull();
        PresetValue = presetValue;
    }

    public ExpressionContext(CodeExpression expression, Type expressionType, object owner) : this(expression, expressionType, owner, null)
    {
    }

    /// <summary>
    ///  The expression this context represents.
    /// </summary>
    public CodeExpression Expression { get; }

    /// <summary>
    ///  The type of the expression. This can be used to determine if a
    ///  cast is needed when assigning to the expression.
    /// </summary>
    public Type ExpressionType { get; }

    /// <summary>
    ///  The object owning this expression. For example, if the expression
    ///  was a property reference to button1's Text property, Owner would
    ///  return button1.
    /// </summary>
    public object Owner { get; }

    /// <summary>
    ///  Contains the preset value of an expression, should one exist.
    ///  For example, if the expression is a property reference expression
    ///  referring to the Controls property of a button, PresetValue will
    ///  contain the instance of Controls property because the property is
    ///  read-only and preset by the object to contain a value. On the other
    ///  hand, a property such as Text or Visible does not have a preset
    ///  value and therefore the PresetValue property will return null.
    ///  Serializers can use this information to guide serialization.
    ///  For example, take the following two snippets of code:
    ///  Padding p = new Padding();
    ///  p.Left = 5;
    ///  button1.Padding = p;
    ///  button1.Padding.Left = 5;
    ///  The serializer of the Padding class needs to know if it should
    ///  generate the first or second form. The first form would be
    ///  generated by default. The second form will only be generated
    ///  if there is an ExpressionContext on the stack that contains a
    ///  PresetValue equal to the value of the Padding object currently
    ///  being serialized.
    /// </summary>
    public object? PresetValue { get; }
}
